You delegate your ADA from one of the supported wallets which currently are: Daedalus and AdaLite. See our "HOWTO" section for step by step instructions how to properly use them.
Daedalus: Full node wallet
AdaLite: Light wallet (Supports staking using Ledger and Trezor hardware wallet)
Yoroi: Light wallet (Staking support coming soon)
Your ADA will never leave your wallet. This said you ADA are always as safe as your wallets are. Make sure to use strong password, store your backup phrases securely offline or ideally use a hardware wallet. You never need to send your ADA to a 3rd party. If you are asked to do so by a website or tool: It's a scam!
The rewards depend on the pool's stake your are delegating to, the owner's pledge, pool's performance and more factors. The higher the pool's stake the more frequently it will create blocks thus a more constant flow of rewards assuming the pool is online and "on tip" when it's his turn to create the block. During the "Incentivised Testnet" lifetime rewards of >12% per year were possible. During mainnet this is expected to be around 4.5 - 5% per year. There is a level at which point the pool hits saturation and rewards will get reduced. Currently this barrier sits at 220 Mio ADA.
You can switch stakepools at any time by choosing a new pool and delegate towards it. The delegation will instantly get changed in the wallet but only take effect after the current plus the next epoch has ended.